US3548589A - Heat engines - Google Patents

Description

Dec. 22, 1970 E. H. COOKE-YARBOROUGH 3,548,

HEAT ENGINES Filed Jan. 15, 1969 r 2 Sheets-Sheet l Flat Dem 1970 E. H. COOKE-YARBOROUGH 3,548,

HEAT ENGINES Filed. Jan. 15, 1969 2 Sheets-Sheet 2 L g5 g3 1 f2 24 3/ T 7 United States Patent O ABSTRACT OF THE DISCLOSURE A Stirling heat engine has hot and cold variable volume chambers, which are intercommunicating through a regenerator, each formed at least in part by flexible walls.

Separation of the gas displacement and power output movement components of the movable diaphragms of the chambers is facilitated by permitting the whole cold chamber to float and the regenerator, which provides mechanical connection between diaphragms of the hot and cold chambers, to move.

This invention relates to heat engines and is particularly concerned with heat engines operating on the Stirling cycle and adapted, for example, to drive an electric generator or to be driven to operate as a refrigerator.

BACKGROUND OF THE INVENTION The Stirling engine, which is a reciprocating engine, is a very eflicient means of converting heat into mechanical power and vice versa and has been developed in the power region of tens of horse power to an efficiency of the order of 35%.

Such engines employ pistons sliding in cylinders and connecting rods which convert the reciprocating motion of the pistons into a rotary motion.

Thermo-dynamically the Stirling cycle is a closed system and a problem which has been regarded as inherent in the system is that of sealing. One solution has been to provide a seal known as a roll-sock between the connecting rod and the crank case.

Also, although conventional Stirling engines of tens of horse power have been operated at efficiencies of the order of 35%, if it is desired to scale down to a fraction of a horse power, the ratio of the area of the sliding surfaces to the power generated increases and the friction losses become significant in reducing the mechanical efficiency.

SUMMARY OF THE INVENTION The invention provides a Stirling engine wherein variable hot and cold volume chambers, which are intercommunicating through a regenerator, are each formed at least in part by flexible walls capable of repetitive deflection for the life of the engine at the working temperature.

The Stirling engine is the only reciprocating engine in which the working gas is ideally at the same temperature as the cylinder walls at all times. Thus the increase in the cylinder area/volume ratio resulting from scalingdown does not result in an increase in thermal losses from the gas to the cylinder walls. Elimination of sliding friction by use of flexible walls allows this advantage to be exploited at low power levels.

In one form of engine according to the invention the hot and cold chambers each have opposed side walls which are relatively movable by virtue of the said flexible Walls, one of the opposed side walls of each chamber is fixed, and coupling between the movable side walls and two electromechanical transducers is arranged for providing output electrical power from the engine and for im- Patented Dec. 22, 1970 posing the necessary quadrature phase relationship between movements of the movable side walls.

Conveniently the coupling for imposing the said quadrature phase relationship between the movements of the movable side walls comprises an electrical coupling between the two transducers, which electrical coupling includes a reactive load.

Preferably the coupling for imposing the said quadrature phase relationship between the movements of the movable side walls comprises a mechanical linkage for providing two separated movements corresponding respectively to the in-phase component of movement of the movable side walls, which component is coupled to a displacer transducer, and to the out-of-phase component of movement of the movable side walls, which component is coupled to a power output transducer, and means for electrically feeding a fraction of the power output, with appropriate phase shift, into the said displacer transducer.

In a preferred form of engine according to the invention the hot and cold chambers each have opposed side walls which are relatively movable by virtue of the said flexible walls, one of the opposed side walls of one of the chambers, preferably the hot chamber, is fixed and the other of the opposed side walls of that chamber is mechanically connected via the regenerator to one of the opposed side walls of the other chamber, so that the regenerator and the side walls to which it is mechanically connected can oscillate bodily to act a a displacer system.

In thihs arrangement output power is derived from the movement of the other side wall of the said other chamber, that is the side wall not directly mechanically connected to the regenerator, relative to the said fixed side wall of the said one chamber. Some means is required to drive the displacer and this could be an electrical means such as an external solenoid. Alternatively the displacer may be driven with the necessary phase and amplitude by making one of the displacer diaphragms slightly larger than the other and by suitable mechanical tuning of the oscillating displacer system. The electrical drive may be preferable in instances where several engines are required to operate in synchronism.

In a further alternative arrangement according to the invention a mechanical coupling is provided between the displacer system and the side wall from which output power is derived so as to provide power with the necessary phase and amplitude for driving the displacer system. The hot and cold chambers may comprise capsules of construction similar to aneroid capsules.

A feature of the invention is that the hot and cold volume chambers can readily be in the form of thin disc shaped cavities in which the heat transfer characteristics are such that the ideal isothermal conditions are closely approached.

The invention may with advantage be used with a radioisotope heat source thus providing self contained very long life mechanical or electrical power generator. Moreover, if a comparatively large diameter thin capsule is employed to define the hot volume, a dilute or mixed fission product source in contact with the whole of one side of the capsule becomes a practicable possibility.

A moving coil system is an example of a linear electric transducer which may be driven by the engine. Other types which may be used with advantage are moving iron and piezo-electric transducers.

Two Stirling engines in accordance with the invention may be mounted back to back with their mechanical terminals connected together to provide a heat pump.

DESCRIPTION OF PREFERRED EMBODIMENTS Specific constructions of Stirling engine embodying the invention will now be described by way of example 3 and with reference to the accompanying drawings in which:

FIG. 1 is a diagram illustrating an engine and generator,

FIG. 2 is a diagram similar to FIG. 1 of a modification,

FIG. 3 is a diagrammatic longitudinal section of a further arrangement,

FIG. 4 is a diagrammatic representation of a modification of the engine of FIG. 1, and

FIG. 5 is a diagrammatic representation of a modification of the engine of FIG. 2 or 3.

Referring to FIG. 1 the engine comprises capsules H and C enclosing the hot and cold volumes respectively of the engine. The hot capsule H has a circular flexible metal diaphragm 1 and the cold capsule C has a similar diaphragm 2. The two volumes are connected by a gas transfer conduit 3 in which is housed a regenerator 4.

Each of the diaphragms 1 and 2 is ocnnected by a rod 5 to a moving coil 6 operating in the annular gap of a pot magnet 7 similar to the magnet and coil assembly of a moving coil loudspeaker. Thus, if the diaphragms 1 and 2 oscillate they drive the respective coils 6 and an alternating current is generated in each coil. For the Stirling cycle, the diaphragms are required to operate approximately 90 out of phase and this is achieved by suitable electrical interconnection of the moving coils including a reactive load. The masses and restoring forces of each moving system are chosen such that their natural frequency of oscillation is compatible with desired frequency of the electrical output. It is also necessary to provide a capacitative component in the load which will, in effect, be equivalent to a mass and will resonate with the gas elasticity at the frequency of operation.

The engine operates to maintain the oscillations if heat is continuously applied from a source to the capsule H and extracted by a sink from the capsule C. When the diaphragms are oscillating 90 out of phase, gas is displaced to and fro between the hot capsule H and the cold capsule C and there is a cyclic change of temperature and volume. The gas does work, which is given up to the electrical load (not shown) by expanding nearly isothermally in the hot capsule Hand abstracting heat from the source. It will be noted that each diaphragm 1 and 2 functions both to displace the gas and to generate power.

In practice it is desirable to position the electrical generating components at one end away from the heat source. This could be achieved by connecting the hot diaphragm to a linkage extending from the cold end but this has the disadvantage of providing an unwanted heat conduction path. An alternative arrangement is shown in FIG. 2.

In FIG. 2 the displacing and power functions of diaphragms are separated. A power diaphragm 8 of a cold capsule C1 is connected through a rod 5 to a moving coil 6 and magnet 7 similarly to FIG. 1. The cold capsule C1 also has a displacer diaphragm 9 to which one end of the gas transfer conduit 3, containing the regeneraltor 4, is connected. The other end of the conduit 3 is connected to a displacer diaphragm 10 of a hot capsule H1. Extraction of heat from the cold capsule C1 is facilitated by a heat sink 11.

The assembly comprising the transfer conduit 3 and diaphragms 9 and 10 thus moves and functions as a displacer, causing the working gas to flow to and fro through the regenerator 4.

Since ideally no work is done on the gas by the displacer assembly and in practice very little, the mass and restoring forces can be chosen to allow it to oscillate freely near its natural frequency and by making the diaphragms 9 and 10 of slightly unequal area the periodic pressure changes due to the gas displacement will furnish the small amount of energy required to keep the assembly in oscillation. The power diaphragm 8 and the moving coil 6 respond to the pressure changes and are also tuned mechanically and electrically to the same frequency and it can be shown that the displacer will oscillate with a phase displacement of approximately relative to the power diaphragm. This phase angle is not critical, however.

In this embodiment it is preferred to resonate the gas elasticity with a suitable mass on the rod 5 which has the advantage over the capacitative loading of FIG. 1 in avoiding electrical losses in the moving coils.

The embodiment shown in FIG. 3 operates on a similar principle to that of FIG. 2 but incorporates a number of modifications and improvements. The hot capsule H2 consists of a rigid plate 11 formed on one side with a cavity 12 closed by a plane flexible diaphragm 13 clamped to the plate 11 by a ring 14. The centre of the diaphragm 13 is supported on each side by rigid discs 15 and 16 having central holes in which the transfer conduit 3 is mounted. The peripheries of the cavity 12 and disc 15 correspondingly chamfered such that when the diaphragm is flexed inwards the cavity is substantially wholly filled by the disc.

The cold capsule C2 is constructed similarly to the hot capsule H2 and is secured to the other end of the transfer conduit 3. Corresponding reference numbers have been given to the similar parts of the hot and cold capsules. The diameter of the cold diaphragm 13a is, however, smaller than that of the hot diaphragm 13 to provide the displacement energy as explained above. A shaft 17 is secured to and extends from the centre of the plate 11a of the cold capsule C2 and this shaft 17 is secured to bushes 18 carried by flexible spiders 19 supported from frame members 20 to which the hot capsule H2 is also secured. Thus the cold capsule C2 as a whole is flexibly mounted to oscillate along its axis and the reciprocating mechanical movement of the shaft 12 may be used to derive an energy output. As in the embodiment of FIG. 2, the mass of said shaft 12 is arranged to resonate at the operating frequency with the gas elasticity.

The mass of the cold capsule C2 provides only a limited heat sink and itself requires to be cooled. This is achieved by means of two apertured plates 21 and 22 fixedly mounted on the frame members 20 parallel with the disc 16a and back plate 11a respectively but just out of contact in the extreme positions of movement of these parts of the capsule C2.

The elfect of the plates 21 and 22 is alternately to suck in and blow out air from the gap between themand the respective parts of the capsule as the parts oscillate, thus providing a degree of forced cooling.

In operation, since the hot capsule diaphragm is larger than the cold capsule diaphragm, when the pressure is high there is a force tending to drive the displacer assembly towards the cold capsule and the contained gas towards the hot cavity. The mass of the displacer assembly and the restoring forces of the diaphragms are chosen to make the system resonant around cycles/sec. which is the chosen operating frequency of the engine. The resistance to movement of the displacer assembly at that frequency is in phase with the gas velocity, which leads the displacer velocity by 45. The force driving the displacer is the gas pressure which is nearly in phase Withl the movements of the output shaft 5. With the displacer tuned to resonance, the driving and resistance forces are:

in antiphase and thus the required 45 relationship is;

displacement is about cc. For a temperature difference of 340 C. the designed power output is 6 watts.

Extrapolation from data on possible diaphragm, materials indicate that diaphragms of a thickness of about 0.33 mm. made of Nimonic 80A or Inconel X should be capable of at least 10 reversals (5 years at 50 c.p.s.) at a temperature as high as 600-700 C. and at stresses at least 50% greater than those assumed for the above diaphragm dimensions.

Power output can be substantially increased by operating with the gas at high pressure say 100 atmos., but the diaphragm deflection would need to be substantially reduced. The necessary displacement could then be achieved by the use of diaphragms in series.

Heat losses along the walls of the transfer conduit 3 require to be minimised and therefore the walls of the conduit are made as thin as possible consistent with standing the axial force on the regenerator cross-section corresponding, in the case of FIGS. 2 and 3, to the gas pressure on the effective area of the diaphragms.

The regenerator is required to be of high efficiency and to present the minimum of fluid friction. It may, for example, consist of a plurality, say 50, of stainless steel wire mesh discs 23 mounted within the conduit 3 normal to its axis. A mesh of .4 mm. diameter wire at 2.5 mm. pitch would be suitable. In the drawing a few of such discs are shown diagrammatically only for the sake of clarity.

In a modification the conduit 3 is replaced by a plurality of conduits each containing a regenerator and dispersed over the area of the rigid central discs and 16 of the diaphragms.

In the above described embodiments the masses are unbalanced and the engine requires to be mounted on a massive base to take up the reaction. Although the massive shielding required of a ratio-active isotope heat source could be used towards this end, it is preferred to provide a balanced system of masses thereby substantially eliminating any external reaction. Thus, in a preferred embodiment, the mass of the moving parts of the capsules, output shaft and the moving components of the transducer are reduced to a minimum and the masses required for mechanical tuning are paired and arranged by means, for example, of suitable linkages, to move in unison in opposite directions thus cancelling any out-of-balance forces.

Additionally, or alternatively, the masses required to produce resonance in the displacer and/or output system may be reduced by introducing an element of negative elasticity to counteract the natural elasticity of the sysd terns. Such an element may consist of a leaf spring prestressed in compression and adapted to move from a central unstable straight condition to a bowed condition in either direction away from the central position.

The engine of the foregong examples has particular advantages when adapted to operate with small oscillatory amplitude but in the case of a moving coil transducer a higher efficiency may be obtainable at higher amplitudes. It may thus be desirable to provide a velocity changing means between the engine and the transducer. If a linkage is provided for mass balancing purposes, this linkage may be readily adapted to provide the required velocity change. Alternatively the moving coil of the transducer may be connected directly through an elastic device such as a helical spring to the engine and the coil provided with the requisite inertial mass to give the desired amplitude.

FIG. 4 illustrates a modification of the engine shown in FIG. 1. The two diaphragms 1 and 2 move move approximately 90 out of phase. The motions can be resolved into two components, an in-phase component which corre sponds to the displacement of the gas between the two cavities at constant volume, and an out-of-phase component which corresponds to a change in the sum. of the two cavity volumes.

In the arrangement of FIG. 1 as described above, these components are resolved electrically. Output power is derived from the out-of-phase component (volume component) and a proportion of this is fed back into the inphase component (displacement component) to maintain oscillation. Electrical resolution in this way involves problems of efficiency and power handling capacity of the transducers and FIG. 4 illustrates an arrangement in which these problems are avoided by resolving the com- I ponents mechanically.

The diaphragms 1 and 2, and two electromechanical transducers, namely a displacement transducer 21 and a power output transducer 22 are mechanically interconnected by a system of levers and links 23, 24, 25, 26 pivoted at the points, marked by a cross, at 27, '28, 29 30, 31, 32, 33, 34, 35, 36. In this example, all the pivots are provided by flexures. Pivots 28 and 35 are fixed to a support frame, as are the bodies of the transducers 21 and 22.

The mechanical resolution of the in-phase and out-ofphase components of the movements of the diaphragms may be appreciated by considering firstly, movement of the displacement transducer 21 with the power transducer 22 locked; this will cause movement of both diaphragms in the same direction corresponding to displacement of gas, and secondly, movement of the power transducer 22 with the displacement transducer 21 locked; this will cause the diaphragms to move in opposite directions which corresponds to a change in total gas volume.

It should be noted that neither motion is entirely independent. Movement of the displacement transducer 21, even with the two cavities at the same temperature, will generate a gas resistance force at the pivot 29 which will be transmitted to the power transducer 22. Movement of the power transducer 22 with the displacement transducer locked, which causes an up and down movement of the pivot 30, will also cause some rotation about this pivot 30 if the displacement transducer lever 24 is of less than infinite length. This would introduce a small amount of displacement into the volume-change movement.

This arrangement of FIG. 4 will operate if some of the power output from the power transducer 22 is suitably phase shifted and fed to the displacement transducer 21. This arrangement would not be subject to the powerhandling and efiiciency difficulties mentioned above, because the amount of power involved in displacing the gas is a relatively small proportion of the total power output. There are, however, two quite simple Ways of coupling the volume and displacement motions mechanically to obtain the desired phase relationship.

The displacement force should be approximately in phase with the gas pressure and in a sense tending to force the gas towards the hot cavity. One method is to put the pivots 30 and slightly out of vertical line, 30 being to the left 35 in FIG. 4. A positive pressure in the two cavities will then have the effect of producing a net force tending to drive both diaphragms upwards, which has the desired effect of driving the gas towards the hot cavity. The same effect can be achieved by mass-loading the cold diaphragm. The acceleration of pivot 30 is proportional to the gas pressure; this added unbalanced mass will produce a couple about pivot 30- proportional to acceleration, which is in the right sense to force the gas towards the hot cavity.

There is a possible refinement of the arrangement of FIG. 4 to reduce the mechanical forces on the walls of the regenerator which connects together the hot and the cold cavities mechanically. To achieve this the pivot 35, instead of being connected directly to the main structure of the assembly is attached to the centre of a bridge, the left-hand end of which is supported on the stationary part of the hot cavity and the right-hand end of which is supported on a stationary pillar close to the vertical link 25.

The foot of this pillar is fixed to the base of the structure. For this arrangement the upper rocker lever 26 and the bridge on which it is supported centrally at 35, act like a scissors configuration, resulting in a nearly exact balance between the forces applied to the hot cavity, and so largely relieving the regenerator of stress due to the pressure force on the hot face.

With the moving regenerator configuration of FIGS. 2 and 3, in principle, a force of the required magnitude and phase to drive the displacer system can be obtained by making the cold diaphragm smaller in area than the hot diaphragm. In practice, however, this may be inconvenient for several reasons. First, the cold diaphragm is in any case more highly stressed than the hot one, so for a given power output its diameter cannot very well be reduced. This means that the hot diaphragm must be enlarged, and more lightly stressed, making the hot face larger than would otherwise be necessary. The second disadvantage is that the relative diaphragm areas have to be designed into the system from the start and there is little or no prospect of adjustment subsequently. Third, for purely practical manufacturing reasons it is much more convenient to have hot and cold diaphragms identical in size.

With equal-size diaphragms the necessary force can be applied to the displacer system using a linkage as shown in FIG. 5. Two ore more radially-disposed levers 41, 42 are connected via flexures 43, 44 at their outer ends to the displacer system and via flexures 45, 46 at their inner ends to the outjut shaft. Cold face plate 47 bears on intermediate points 48, 49. The force applied at 48, 49 is directly proportional to the gas pressure. The proportion of this force which is applied to drive the displacer is a/ b. The remainder is applied to output shaft 51. Clearly by suitable choice of the position of points 48, 49 any desired force can be applied to the displacer and this force is directly in phase with gas pressure. A tuning mass is indicated at 52 and part of the regenerator is shown at 53.

The invention is not restricted to the details of the foregoing examples.

I claim:

1. A Stirling engine comprising hot and cold variable volume chambers inter-communicating through a regenerator, each said chamber being formed at least in part by flexible walls capable of repetitive deflection for the life of the engine at the working temperature, non-positive coupling means between side portions of the hot and cold chambers, said side portions being movable by virtue of the flexible walls, the coupling means transmitting force for maintaining reciprocating gas displacement between the chambers, and the operating components of the en gine being tuned to resonate in correct phase relationship in response to the forces transmitted by the coupling means.

2. A Stirling engine as claimed in claim 1, wherein the hot and cold chambers each have opposed side portions which are relatively movable by virtue of the said flexible walls, one of the opposed side portions of one of the chambers is fixed and the other of the opposed side portions of that chamber is mechanically connected through the regenerator to one of the opposed side portions of the other chamber, so that the regenerator and the side portions to which it is mechanically connected can oscillate bodily to act as a displacer system.

3. A Stirling engine as claimed in claim 2 wherein, when the engine is operated to convert heat into mechanical energy, output power is derived from the movement of the other side portion .of the said other chamber not directly mechanically connected to the regenerator, relative to the said fixed side portion of the said one chamber.

4. A Stirling engine as claimed in claim 2, wherein the said one chamber is the hot chamber, when the engine is used for converting heat into mechanical energy and is the refrigerated chamber when the engine is used as a refrigerator.

5. A Stirling engine as claimed in claim 4, wherein the area of the relatively movable opposed side portions of the said other chamber is less than the area of the relatively movable opposed side portions of the said one chamber, whereby gas pressure in the chambers provides the aforesaid coupling means for transmitting force for mainraining reciprocating gas displacement between the chambers.

6. A Stirling engine as claimed in claim 2, wherein a mechanical lever coupling is provided between the displacer system and the side portion the movement of which comprises the mechanical power movement, the mechanical lever coupling providing the aforesaid coupling means for transmitting force for maintaining reciprocating gas displacement between the chambers.

7. A Stirling engine as claimed in claim 1, wherein the movable side portions of the hot and cold chambers are each coupled to an electromechanical transducer for providing output electrical power from the engine.

8. A Stirling engine as claimed in claim 7, wherein the said coupling means for transmitting force for maintaining reciprocating gas displacement between the chambers comprises an electrical coupling between the two transducers, which electrical coupling includes a reactive load.

9. A Stirling engine as claimed in claim 7, wherein the said coupling means for transmitting force for maintaining reciprocating gas displacement between the chambers comprises a mechanical linkage for providing two separated movements corresponding respectively to the inphase component of movement of the movable side portions, which component is coupled to a displacer transducer, and to the out-of-phase component of movement of the movable side portions, which component is coupled to a power transducer, and means for electrically feeding a fraction of the power transducer power, with appropriate phase shift, into the said displacer transducer.

10. A Stirling engine as claimed in claim 1, wherein at least one of the chambers comprise capsules of construction similar to aneroid capsules.

11. A Stirling engine as claimed in claim 1, wherein the chambers comprise thin disc-shaped cavities.

References Cited UNITED STATES PATENTS 2,549,464 4/1951 Hartley 290-1 2,611,236 9/1952 Kohler et a1. 60 24 2,907,169 10/1959 Newton Q 6024 3,232,045 2/1966 Fokker 6024 3,339,077 8/1967 Shapiro 290-1 MARTIN P. SCHWADRON, Primary Examiner R. BUNEVICH, Assistant Examiner U.S. Cl. X.R. 6'2-6; 290 1

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